Plans for Syrian peace talks in Geneva next month appeared in danger of being derailed on Wednesday night as the country's divided opposition movement issued a fresh demand for Bashar al-Assad's government to be excluded from the political process, while Damascus insisted the Syrian president would stay in power until 2014 and possibly beyond.

Russia and France also clashed over whether Iran should be allowed to attend the talks, and diplomats suggested that the mid-June target date might have to be pushed back. Turkey warned that if the negotiations failed, it would mark the end of the road for diplomacy and open the gates to the wholesale arming of opposition forces.

In the midst of fierce internal wrangling in Istanbul, the fragmented National Coalition for Syrian Revolutionary and Opposition Forces issued its first statement on the Geneva conference, but did not say it would attend. Instead it laid down a list of demands, for "the head of the regime, security and military leadership to step down and be excluded from the political process" and for a "halt to Iran and Hezbollah's invasion of Syria and [their expulsion] from the country". The coalition said a timetable for these goals was needed "with the international community offering binding guarantees".

The Assad government has said it will attend the talks but the foreign minister, Walid al-Moallem, gave an interview to a Lebanese television station in which he laid down a hard line.

"Our armed forces have regained the momentum," he said. "From now until the next elections, President Bashar Assad is president of the Syrian Arab Republic.

"Will Assad run in 2014 or not? This depends on the circumstances in 2014 and on the popular will."

While the opposition and Damascus sought to frame the talks with conflicting set outcomes, a potentially far more serious hurdled loomed in the shape of an international row over the participation of Iran, which is an active party to the conflict, sending Revolutionary Guards to train pro-government militias. It also backs Hezbollah, which has taken a key role on the government side of the war.

Russia is adamant that an Iranian delegation should be present. "The issue of Iran is key for us," the Russsian foreign minister, Sergei Lavrov, was quoted as saying by his country's news agencies. "Iran without question is one of the most important nations." Tehran has voiced readiness to attend and Turkey has backed the proposal.

Saudi Arabia and France have led opposition to an Iranian presence. Riyadh has said it will boycott Geneva if Tehran takes part, while the French foreign minister, Laurent Fabius, said that to include Iran would be "extremely dangerous" as it would harm prospects of reaching a deal on its disputed nuclear programme.

"We fear that if they are part of the Syrian conference they will try to drag things on to such an extent that they will blackmail us saying that the Syrian crisis can only be resolved on condition that they have the nuclear bomb," Fabius told France Inter radio. Tehran denies having any aspirations to build a nuclear weapons.

The US state department noted that Iran had not played a constructive role in Syria, but that its attendance was a matter for negotiation.

Spokeswoman Jen Psaki said the decision "will be made in cooperation and through discussion between the US, Russia, our international partners and the United Nations. That decision has not yet been made."

UN headquarters in New York is keen to see Tehran involved in the talks. "Iran should be there, but the resistance is very strong," a senior UN official said. "If we can't get Iran to the talks, we will need a parallel channel."

Russian officials have warned that the lifting of the EU arms embargo, due to take effect on Saturday, also undermines prospects for the talks. In a further sign of mounting friction in the runup to the Geneva meeting, Lavrov also took aim at a US-backed resolution at the UN human rights council condemning the Syrian government. He described the resolution as odious and unwholesome.

A senior UN official involved in preparing the talks told the Guardian that the biggest problem was getting the right people from both sides of the conflict to attend. "We need credible negotiating partners. This is the most important issue. This is not yet solved. The first condition concerns the quality of the talks," he said.

Omer Celik, the foreign affairs chief in Turkey's ruling AKP party, portrayed the Geneva meeting as a last chance for the international community to salvage its credibility in the face of two years of slaughter in which more than 80,000 people have died.

"If the Geneva meeting becomes just another means of gaining Assad some additional time, that will mean that all diplomatic initiatives will have been exhausted," Celik, who is also Turkey's culture minister, told the Guardian in Ankara.

"If the Geneva meeting does fail, we believe the international community should take all necessary measures to support the protection of opposition forces," Celik said. Asked if that meant arming the opposition, he replied: "Of course."

The White House backed the Syrian opposition's insistence that the outcome of the negotiation process must involve the end of the Assad government, but it was non-committal on what action it would take if talks fail to reach that goal.

"We are committed to our support of the Syrian opposition and committed to reassessing the options available to the president when it comes to future action in Syria and whether that would bring us closer to peaceful transition," the White House spokesman, Jay Carney, said. He played down recent reports that the administration was reassessing the possibility of enforcing a no-fly zone.

"Every option remains on the table. That of course includes a no-fly zone. It is inaccurate to suggest that option was in development only now. The suggestion that there is some new option [for a no fly zone] does not seem newsworthy."